As a method for joining metal members to each other, friction stir welding (FSW) is known (refer to Documents 1 to 9 (Japanese Published Unexamined Patent Applications published by the Japan Patent Office) listed below). According to friction stir welding, a rotating tool is moved along a butting portion between metal members while the rotating tool is rotated, and by plasticizing and fluidizing the metal of the butting portion by frictional heat of the rotating tool and the metal members, the metal members are joined in solid phase. The rotating tool is generally formed by providing a stirring pin (probe) in a projecting manner on a lower end surface of a shoulder having a columnar shape.